If they're carrying significant current (not exactly the same as being live of course!) - they would be a little warmer than ambient - a couple of thermometers and a bit of thermal insulation might give an indication.

On the other hand - what would you do if you knew for certain they were live? If the plan is just to leave them alone, then is there any harm is just presuming that they're live?

If the ends aren't accessible - would there be a question over the ownership of the cable?

Sufficient inteference will get onto the outer sheath for it to be picked up by a CAT. A 50Hz signal is very good at getting on to anything metallic and there will be sufficient to pick up from a cable sheath whether it is live or dead. The only way to positively identify a cable is using signal injection, but that usually involves access to both ends of the cable as well as making it dead.